This invention relates to a transport mechanism to carrying a sheet of recording paper to a desired position in an imaging device, employing a so-called electrophotographic imaging process, and more particularly to a transport mechanism to transport a sheet of recording paper to which a toner image has been transferred but has not yet been fixed from a transfer unit to a fixing unit.
Imaging devices employing the electrophotographic imaging process are known in the art, such as electronic copying machines and laser printers, in which a uniformly charged surface of a photoconductive drum undergoes exposure to build a certain latent image. This latent image is then powdered with toner for development of a toner image, which is transferred onto a sheet of recording paper and permanently fixed to it by means of a fixing unit. The sheet of recording paper is then discharged out of the imaging device.
Referring to an example of a laser printer illustrated in FIG. 25, the imaging device employing the electrophotographic process comprises a drum 1 that is coated on its surface with a photoconductive material and a charger unit A, a exposing unit B, developing unit C, transfer unit D, cleaning unit E and decharger unit F, all mounted around the drum 1 in the order illustrated order in the rotational direction of the drum 1. The photoconductive material coated on the drum 1, which is uniformly charged by the charger unit A, is exposed to laser beams carrying image information by means of the exposure unit B, forming a latent image thereon. The toner is then adhered to the latent image by the developing unit C. The transfer unit D then transfers the resultant toner image formed on the drum 1 to a sheet of recording paper P, which is transported in synchronism with the rotation of the drum 1. The toner image is fused onto the paper P by means of the fixing unit G, the paper then being allowed to leave the imaging device.
The sheet of recording paper P to which the toner image has been transferred by the transfer unit D is led to the fixing unit G by means of a guide member or transport mechanism, not shown. Since the paper being conveyed from the transfer unit D to the fixing unit G carries the transferred toner image that remains unfixed, the guide member or transport mechanism is configured to guide or hold the back (the side wherein no toner image is deposited) of the recording paper P.
In case that the toner image is transferred to the up side of the recording paper P and the paper P is discharged with the same side up (called a face-up discharge system), it is easy to guide the paper P along the paper feed path by holding the back (down) side of the paper P, on which no toner image is deposited.
In order to develop a color image employing a similar electrophotographic process, there are so-called color copiers or similarly configured printers available. In such an imaging device, an original image is separated into three primary color images by means of red, green and blue filters. The separated color images are developed by colored cyan, magenta and yellow toners and are transferred to the recording paper so that they are laid down one upon another, thus providing a duplicate of the same colors as the original image. In such color imaging devices, the transfer process must be repeated several times on the same area of the same recording paper (as many times as there are toner colors). To simplify this process, a transfer drum is provided adjacent to the photoconductive drum for a synchronized rotation with the latter, so that the toner image on the photoconductive drum is transferred to the surface of the paper wrapped around the circumferenctial surface of the transfer drum.
However, especially for continuously feeding multiple sheets of paper, as in printers, a so-called face-down discharge system, with which paper sheets are discharged with their image-deposited sides down, is desired to enable the discharged sheets of paper to be stacked in the original page number order (The sheets of paper are stacked in the reversed order of page number in the face-up discharge system).
Such a face-down discharge process is made possible by an arrangement of the operating parts (charger, exposure, development, transfer, cleaning and decharger units) such that the image is transferred to the down side of the paper at the upper area of the drum surface, and the paper with the image deposited on the down side is discharged out of the device as it is. In such an arrangement, however, the toner image deposited on the down side of the paper prohibits the transport system from holding the down side of the paper in the paper feed path from the transfer unit to the fixing unit. As a result, it is hard to appropriately guide the paper in the correct transport direction, making it difficult to bring the paper into a correct position at the fixing unit.
One solution to this problem is to use the same arrangement of the drum and the parts around the drum as for the face-up discharge system, while turning the recording paper up side down in the paper feed path from the fixing unit to the discharge unit to allow the paper to be discharged with its face down. Such an arrangement, however, has a problem in that the paper feed path must be longer and more complicated, resulting in more frequent paper clogging in this path and an increased size of the device.
In the case of the color imaging device, in which several transfer processes take place on the recording paper wrapped around the transfer drum, there has been a problem in that the paper wrapped around the drum remains curled after separating from the drum, making a proper guiding control of the paper in a predetermined direction difficult. There are some devices in which a metal guide plate provided with a number of perforations are placed along the paper feed path so that suction air sent from a blower behind the guide plate applies a drawing force to the recording paper. With this structure, the recording paper is drawn to the guide plate to straighten out the curl, allowing the paper to travel straight along the guide plate. The guiding direction of the paper can be adjusted to a certain extent by changing the setting of the guide plate, ensuring a greater freedom in design choice. However, this structure (using the blower for the paper feed) requires extra mechanical parts, including the blower and its piping and is associated with other problems, such as interference with the parts of the fixing unit and so on, heat generated from the blower and power supply capacity. Providing solutions to the aforementioned problems again leads to a larger size and greater complications of an overall device and therefore an increased cost.